Equality of two structs in C#

Question

I look for an equality between two instances of this struct.

public struct Serie<T>
{
    T[]         X; 
    double[]    Y;

    public Serie(T[] x, double[] y)
    {
        X = x;
        Y = y;
    }

    public override bool Equals(object obj)
    {
        return obj is Serie<T> && this == (Serie<T>)obj;
    } 
    public static bool operator ==(Serie<T> s1, Serie<T> s2)
    {
        return s1.X == s2.X && s1.Y == s2.Y;
    }
    public static bool operator !=(Serie<T> s1, Serie<T> s2)
    {
        return !(s1 == s2);
    }

This doesn't work. What am I missing?

        double[] xa = { 2, 3 };
        double[] ya = { 1, 2 };
        double[] xb = { 2, 3 };
        double[] yb = { 1, 2 };
        Serie<double> A = new Serie<double>(xa, ya);
        Serie<double> B = new Serie<double>(xb, yb);
        Assert.AreEqual(A, B);

Answer 1

You're comparing the array references rather than their contents. ya and yb refer to different arrays. If you want to check the contents of the arrays, you'll have to do so explicitly.

I don't think there's anything built into the framework to do that for you, I'm afraid. Something like this should work though:

public static bool ArraysEqual<T>(T[] first, T[] second)
{
    if (object.ReferenceEquals(first, second))
    {
        return true;
    }
    if (first == null || second == null)
    {
        return false;
    }
    if (first.Length != second.Length)
    {
        return false;
    }
    IEqualityComparer comparer = EqualityComparer<T>.Default;
    for (int i = 0; i < first.Length; i++)
    {
        if (!comparer.Equals(first[i], second[i]))
        {
            return false;
        }
    }
    return true;
}

As an aside, your structs are sort of mutable in that the array contents can be changed after the struct is created. Do you really need this to be a struct?

EDIT: As Nick mentioned in the comments, you should really override GetHashCode as well. Again, you'll need to get the contents from the arrays (and again, this will cause problems if the arrays get changed afterwards). Similar utility method:

public static int GetHashCode<T>(T[] array)
{
    if (array == null)
    {
        return 0;
    }
    IEqualityComparer comparer = EqualityComparer<T>.Default;
    int hash = 17;
    foreach (T item in array)
    {
        hash = hash * 31 + comparer.GetHashCode(item);
    }
    return hash;
}

Answer 2

I don't think there's anything built into the framework to do that for you, I'm afraid

In 4.0, there is:

StructuralComparisons.StructuralEqualityComparer.Equals(firstArray, secondArray);

Answer 3

You should compare the contents of the Array in your Equality logic ...

Also, it is recommended that you implement IEquatable<T> interface on your struct, as this prevents boxing/unboxing issues in some cases. http://blogs.msdn.com/jaredpar/archive/2009/01/15/if-you-implement-iequatable-t-you-still-must-override-object-s-equals-and-gethashcode.aspx

Answer 4

The part s1.Y == s2.Y tests if they are 2 references to the same array instance, not if the contents are equal. So despite the title, this question is actually about equality between array(-reference)s.

Some additional advice: Since you are overloading you should design Serie<> as immutable and because of the embedded array I would make it a class instead of a struct.

Answer 5

Calling == performs reference equality on arrays - they don't compare the contents of their elements. This basically means that a1 == a2 will only return true if the exact same instance - which isn't what you want, I think..

You need to modify your operator == to compere the contents of the x array, not it's reference value.

If you're using .NET 3.5 (with link) you can do:

public static bool operator ==(Serie<T> s1, Serie<T> s2)
{
    return ((s1.X == null && s2.X == null) || s1.X.SequenceEquals( s2.X )) 
           && s1.Y == s2.Y;
}

If you need to do deep comparison (beyond references), you can supply SequenceEquals with a custom IEqualityComparer for the type of T.

You probably should also consider implementing the IEquatable<T> interface for your struct. It will help your code work better with LINQ and other parts of the .NET framework that perform object comparisons.

Answer 6

You can create a private accessor for your struct and use CollectionAssert:

[TestMethod()]
public void SerieConstructorTest()
{
    double[] xa = { 2, 3 };
    double[] ya = { 1, 2 };
    double[] xb = { 2, 3 };
    double[] yb = { 1, 2 };
    var A = new Serie_Accessor<double>(xa, ya);
    var B = new Serie_Accessor<double>(xb, yb);
    CollectionAssert.AreEqual(A.X, B.X);
    CollectionAssert.AreEqual(A.Y, B.Y);
}

This code works fine.

References:

• CollectionAssert.AreEqual Method
• How to create private accessor